1. Field of the Invention
This invention relates to the use of unsaturated derivatives of saturated or highly saturated elastomers as elastomer components in the preparation of synergistically covulcanizable elastomer blend compositions.
2. Description of Related Art
Vulcanizates based on blends of elastomers which contain little or no olefinic unsaturation with more highly unsaturated elastomers are of interest in the rubber industry primarily because of their special properties, e.g., superior resistance to ozone degradation and consequent cracking, improved resistance to chemical attack, improved temperature stability and unique dynamic response. Examples of such blends are those containing: (a) one or a mixture of low unsaturated rubbers such as terpolymers of ethylene, propylene and up to about 10 wt % of a non-conjugated diene (EPDM), copolymers of a C.sub.4 to C.sub.7 isoolefin, e.g., isobutylene, with up to about 20 wt % isoprene (butyl rubber), halogenated butyl rubber and halogenated interpolymers of a C.sub.5 to C.sub.7 isoolefin, e.g., isobutylene with up to about 20 wt % para-alkylstyrene, e.g., para-methylstyrene, mixed with: (b) one or more ethylenically unsaturated elastomers such as natural rubber, polybutadiene, copolymers of butadiene with styrene or acrylonitrile, and the like.
One of the problems associated with the curing of elastomer compositions containing such chemically diverse materials is the inability to achieve a balanced cure of each individual component and also a truly intervulcanized composition, i.e., a composition where predominant interpolymer crosslinking takes place between different polymer molecules in the different phases. For example, in sulfur curable systems containing a blend of highly unsaturated elastomer such as natural rubber and a low unsaturation elastomer such as EPDM or butyl rubber, the high unsaturation diene phase cures much faster than the low unsaturation phase resulting in a highly cured polydiene phase and an undercured, curative starved low unsaturation phase, with little or no interpolymer crosslinking taking place at the phase boundaries. As a consequence of this lack of curing balance, the vulcanizates may exhibit inferior mechanical properties such as low tensile strength, low modulus, poor hysterisis and the like.
One technique used to minimize the problem of vulcanization imbalance is the use of low or no unsaturation blend components which have been modified by the inclusion of functional groups in the polymer chain which functional groups are susceptible to crosslinking mechanisms independent of the sulfur curing system used to crosslink the highly unsaturated elastomer. For example, blends containing halogenated butyl rubber or halogenated interpolymers of isobutylene and para-methylstyrene can be vulcanized along with more highly unsaturated rubbers by including an independent curing system for each type of elastomer into the vulcanization recipe, e.g., a zinc oxide-based curing system which normally cures the halorubber and an accelerated sulfur curing system which normally cures the highly unsaturated rubber. However, even in these systems there may be a drop-off of important properties of the cured composition such as modulus, tensile, elongation and the like in comparison to what can be calculated as the composition-based averaging of such properties based on the properties exhibited by each cured elastomer alone, i.e., the "tie line" properties. This drop-off in properties is most likely the result of the lack of ideal solubility (compatibility) of the different phases coupled with the lack of significant interpolymer crosslinking.
It should be noted that the achievement of tie line or above tie line rheometer cure performance in curable elastomer blend systems is an unusual phenomenon and represents optimum cure performance. In most systems, rheometer torque increase for cured blends of different elastomers will lie at least partially below the tie line which would be graphically depicted as a straight line over the blend range connecting the rheometer torque increase values associated with each individual elastomer if cured alone. One blend system which allegedly achieves above tie line performance is disclosed in Hopper et al., "Ozone Resistant Blends", International Conference on Advances in the Stabilization and Controlled Degradation of Polymers, Lucene, Switzerland, May 23-25, 1984. The publication discloses sulfur curable blends of modified EPDM rubber and a polydiene rubber such as natural or polybutadiene rubber exhibiting blend torque increases which are generally slightly above tie line values. The modified EPDM employed is the addition product of an N-chlorothio-N-methyl-p-toluene sulfonamide to elastomeric terpolymers of ethylene, propylene and a non-conjugated diene such as 1,4-hexadiene or dicyclopentadiene.